The error light flashing slowly on the M221 is a Minor error has been detected and the controller has detected an error but can remain running. This could be caused by a bad I/O card in the configuration.

If SL LED is blinking, this is generally an indication that the PLC is attempting to communicate over the serial port.  The internal program may include serial communication to anther device so the SL LED is used to indicated communication is active.

The user guide for M221 can be found at this link.

https://www.se.com/us/en/download/document/EIO0000000976/?gclsrc=aw.ds&&utm_source=google&utm_medium=cpc&utm_campaign=2025_June_us_serv_sev_google_decision-lead_gen_sem_local_services_ecocare_pmax&utm_term=&gad_source=1&gad_campaignid=22715060821&gbraid=0AAAAADcf1WSEZGodMV9YWVIHTW2tTjwP7&gclid=EAIaIQobChMI257SgK3vkAMVfiytBh22ID2wEAAYASAAEgJCvPD_BwE

You can find information regarding all status LED functions and descriptions in the user guide.

Search the ser guide for " table describes the status LEDs" , to find details on what each LED represents and what the behavior of the LED's mean.

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